The present invention relates to dynamoelectric machines, and in particular, to an improved brush holder assembly that cannot be assembled incorrectly and that can be safely released, retracted, and replaced, while maintaining machine operation.
In dynamoelectric machines, a brush holder assembly transfers current from a stationary brush to a rotating contact, such as a slip ring, which then supplies excitation power to a dynamoelectric machine. The brush is mounted in a brush box, and is slidable radially inward toward the surface of the rotating contact. The brush is maintained on the rotating surface of the slip ring by means of a constant pressure spring mounted on a brush holder. Under spring pressure, the brush wears down and requires replacement for maintenance. Power industry expectation developed over the last 20 years is that worn bush assemblies must be able to be removed while a machine is running and under voltage.
Brush holder assemblies are generally characterized as single brush holder assemblies and magazine or multiple brush holder assemblies. Two patents that disclose prior single cartridge, brush replacement assemblies are U.S. Pat. No. 3,710,478 issued to Krulls in 1973 and U.S. Pat. No. 3,864,803 issued to Ohmstedt in 1975. The Ohmstedt brush assembly is shown in FIG. 1. It includes inwardly biased brush holder legs 4 that are spread apart to allow clamping ramps 5 to release and permit the brush 2 to “float” while in an operational mode. Then for retraction, the legs 4 are released so that the brush 2 can be clamped and removed.
The brush holder assembly shown in FIG. 1 has been, because of reliability, safety, and high cost issues associated with the clamping ramps 5 and legs 4, replaced by the brush assembly shown in FIG. 2. In the brush assembly shown in FIG. 2, the clamping ramp 5 feature for retraction is eliminated in favor of a spool 6 and clip 3 arrangement.
Currently, there are at least three brush holder assemblies that allow worn brush assemblies to be removed while a unit is operating. The brush assembly for single brush holders shown in FIG. 2 has evolved over the past 20 years and has a good reliability record. However, as the power utility industry has become more safety conscious in recent years, complaints have occurred with respect to the flexibility of the brush handle and worn brush assembly during the brush removal process. In addition, as new and inexperienced brush holder suppliers have begun supplying parts, it is now apparent that the brush holder assembly shown in FIG. 2 is not “mistake proof”. For example, a number of instances have occurred where the clip 3 has been installed backwards 180 degrees with respect to the brush 2, causing the clip 3 to hang up on the brush holder 1 at the point of interference 4 shown in FIG. 2, as the brush wears down and clip 3 moves down toward holder 1. The backward installation of clip 3 has lead to a flashover or loss of electrical contact situation between the brush 2 and the slip ring on an operating machine, leading to a forced shutdown of the machine and a rework of the brush holder assembly. Thus, it would be desirable to have a brush holder assembly that eliminates any possibility of brushes being assembled incorrectly.